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Heterometallic triiron-oxo/hydroxo clusters: effect of redox-inactive metals.

ABSTRACT: A series of tetranuclear oxo/hydroxo clusters comprised of three Fe centers and a redox-inactive metal (M) of various charge is reported. Crystallographic studies show an unprecedented Fe3M(?4-O)(?2-OH) core that remains intact upon changing M or the oxidation state of iron. Electrochemical studies reveal that the reduction potentials (E1/2) span a window of 500 mV and depend upon the Lewis acidity of M. Using the pKa of the M-aqua complex as a measure of Lewis acidity, these compounds display a linear dependence between E1/2 and acidity, with a slope of ?70 mV per pKa unit. The current study of [Fe3MO(OH)] and previous ones of [Mn3MOn] (n = 2,4) moieties support the generality of the above relationship between the reduction potentials of heterometallic oxido clusters and the Lewis acidity of incorporated cations, as applied to clusters of different redox-active metals.

SUBMITTER: Herbert DE

PROVIDER: S-EPMC3953215 | biostudies-literature | 2013 Dec

REPOSITORIES: biostudies-literature

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Heterometallic triiron-oxo/hydroxo clusters: effect of redox-inactive metals.

Herbert David E DE Lionetti Davide D Rittle Jonathan J Agapie Theodor T

Journal of the American Chemical Society 20131216 51

A series of tetranuclear oxo/hydroxo clusters comprised of three Fe centers and a redox-inactive metal (M) of various charge is reported. Crystallographic studies show an unprecedented Fe3M(μ4-O)(μ2-OH) core that remains intact upon changing M or the oxidation state of iron. Electrochemical studies reveal that the reduction potentials (E1/2) span a window of 500 mV and depend upon the Lewis acidity of M. Using the pKa of the M-aqua complex as a measure of Lewis acidity, these compounds display a ...[more]

PMID: 24304416

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Project description:Reactions are described that interconvert vanadium(IV) oxo-hydroxo complexes [V(IV)O(OH)(R(2)bpy)(2)]BF(4) (1a-c) and vanadium(V) dioxo complexes [V(V)O(2)(R(2)bpy)(2)]BF(4) (2a-c) [R(2)bpy = 4,4'-di-tert-butyl-2,2'-bipyridine ((t)Bu(2)bpy), a; 4,4'-dimethyl-2,2'-bipyridine (Me(2)bpy), b; 2,2'-bipyridine (bpy), c]. These are rare examples of pairs of isolated, sterically unencumbered, first-row metal-oxo/hydroxo complexes that differ by a hydrogen atom (H(+) + e(-)). The V(IV)-(t)Bu(2)bpy derivative 1a has a useful (1)H NMR spectrum, despite being paramagnetic. Complex 2a abstracts H(*) from organic substrates with weak O-H and C-H bonds, converting 2,6-(t)Bu(2)-4-MeO-C(6)H(2)OH (ArOH) and 2,2,6,6-tetramethyl-N-hydroxypiperidine (TEMPOH) to their corresponding radicals ArO(*) and TEMPO, hydroquinone to benzoquinone, and dihydroanthracene to anthracene. The equilibrium constant for 2a + ArOH <==> 1a + ArO(*) is (4 +/- 2) x 10(-3), implying that the VO-H bond dissociation free energy (BDFE) is 70.6 +/- 1.2 kcal mol(-1). Consistent with this value, 1a is oxidized by 2,4,6-(t)Bu(3)C(6)H(2)O(*). All of these reactions are surprisingly slow, typically occurring over hours at ambient temperatures. The net hydrogen-atom pseudo-self-exchange 1a + 2b <==> 2a + 1b, using the (t)Bu- and Me-bpy substituents as labels, also occurs slowly, with k(se) = 1.3 x 10(-2) M(-1) s(-1) at 298 K, DeltaH(double dagger) = 15 +/- 2 kcal mol(-1), and DeltaS(double dagger) = 16 +/- 5 cal mol(-1) K. Using this k(se) and the BDFE, the vanadium reactions are shown to follow the Marcus cross relation moderately well, with calculated rate constants within 10(2) of the observed values. The vanadium self-exchange reaction is ca. 10(6) slower than that for the related Ru(IV)O(py)(bpy)(2)(2+)/Ru(III)OH(py)(bpy)(2)(2+) self-exchange. The origin of this dramatic difference has been probed with DFT calculations on the self-exchange reactions of 1c + 2c and on monocationic ruthenium complexes with pyrrolate or fluoride in place of the py ligands. The calculations reproduce the difference in barrier heights and show that transfer of a hydrogen atom involves more structural reorganization for vanadium than the Ru analogues. The vanadium complexes have larger changes in the metal-oxo and metal-hydroxo bond lengths, which is traced to the difference in d-orbital occupancy in the two systems. This study thus highlights the importance of intrinsic barriers in the transfer of a hydrogen atom, in addition to the thermochemical (bond strength) factors that have been previously emphasized.

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Heterometallic triiron-oxo/hydroxo clusters: effect of redox-inactive metals.

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Heterometallic triiron-oxo/hydroxo clusters: effect of redox-inactive metals.

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